Depends what country you're from.

http://en.wikipedia.org/wiki/Long_and_short_scales

Personally I'm in the Long scale camp. It makes far more sense. That's why I was confused to not see the 'correct' answer listed :)

For example in the Short scale, their logic is: billion= 1,000 ^ (1 + 2). So for example TRIllion, the tri is pretty unintuitive. It has 3+(3 * TRI) zeros.

In the Long scale, the logic is trillion=1,000,000^TRI or 6 * TRI zeros.

I had no idea the UK had 'officially' abandoned the Long scale in 1970s, while the rest of Europe still officially uses it. I'm sure I was taught Long scale. Shame.